1. Field of the Invention
The present invention relates to a semiconductor memory device. In particular, the present invention relates to a fast non-volatile random access memory (RAM) by utilizing a magneto resistance effect.
2. Description of the Related Art
Expectations have been raised on an MRAM (a magnetic random access memory) as a next generation fast non-volatile memory provided with merits of both of a DRAM and a FLASH, which are typical semiconductor memories for the present.
The MRAM is a fast non-volatile memory by utilizing a tunnel magneto resistance (TMR) of a ferromagnetic spin tunnel junction (MTJ). Since IBM has announced that a memory of 256 M bit will be produced in 2004 in cooperation with Infineon, such a memory has become a focus of attention. On the academic level, Motorola tried to fabricate an array of 1 MB and reported the confirmation of memory operation in 2002 Symposium on VLSI Circuits Digest of Technical Papers, pp. 160–163.
The operating principle of the MRAM will be simply described below. First, a description will be given of both of the MTJ and the TMR as the bases of a memory function. In the MTJ, a thin tunnel insulator film 2 is held between two ferromagnetic layers 1 and 3, as illustrated in, for example, FIG. 2. The tunnel conductance of this structure is proportional to the product of the density of states on a Fermi level of two ferromagnetic materials. FIGS. 3A and 3B illustrate the density of states when the spins of the two ferromagnetic materials are parallel (FIG. 3A) and anti-parallel (FIG. 3B) in comparison with each other. Since the spin direction is held before and after tunneling, a tunnel resistance is small when the spins are parallel; in contrast, it is great when the spins are anti-parallel, as is clear from FIGS. 3A and 3B. As a result, one of the spin directions of ferromagnetic spin tunnel junctions is fixed while the other spin direction is varied by an outer magnetic field, so that hysteresis characteristics illustrated in FIG. 4 are exhibited, thereby providing a memory. A spin flip rate is represented in the order of nsec. Even if no magnetic field is applied, the spin direction is fixed, and therefore, the operation of the fast non-volatile memory can be expected.
FIGS. 5 and 6 illustrate an equivalent circuit of the MRAM which has been commercially available so far, and its cross-sectional structure. Next, explanation will be made on writing and reading operations in the MRAM illustrated in FIGS. 5 and 6. At the time of a writing operation, a current is allowed to flow in a bit line 6 and a word line 7 for write, and then, the spin direction is written at a selected cell by a generated combined magnetic field. At a non-selected cell, since an applied magnetic field is small, the spin direction cannot be varied. At the time of a reading operation, a word line 8 for read is turned on, and then, ‘0’ and ‘1’ are determined based on a current flowing between a common ground line 13 and the bit line 6.
As described above, in the MRAM, the current is allowed to flow in the word line for write and the bit line, and data is written by using the generated combined magnetic field. At this time, since the word line 7 for write is separated from a TMR element in view of a distance, it is, in principle, necessary to allow a large current to flow when the data is written. Consequently, there has arisen a problem that power consumption is large. Moreover, it has been necessary to reduce a write current from the viewpoint of the secureness of migration reliability of a wiring.